A modified cellulose, also referred to herein as a cellulose derivative, is used in plaster and joint compounds as a thickener (or to modify the rheology in some way) and to improve the workability of gypsum-based compounds. Cellulose ethers have been known to improve some other properties, including the consistency, adhesion and water retention of gypsum-based joint compounds and tile adhesives. However, some of these properties, specifically thickening, are not considered beneficial for the production of wallboard. Wallboard is formed from a slurry that is continuously mixed and fed onto a belt. Thus, it is desirable for the slurry used to make wallboard to be thinner than a plaster.
Typically, a small amount, e.g. less than 0.25 wt %, of cellulose ethers is added to the dry ingredients of plaster or joint compound, which may be limestone-based rather than gypsum-based, prior to mixing with water. This tends to improve the strength of the plaster or joint compound somewhat, as well as providing the desired thickening. However, additions of cellulose ethers greater than 0.25 wt %, particularly at high viscosity grade (the viscosity of an aqueous solution of the cellulose ether measured at a 2 wt % concentration of the cellulose ether in water), tend to reduce the strength of gypsum-based products. See Udo Ludwig and N. B. Singh, Il Cemento, v.1 (1979) 39-50, and Felix Brandt and Dirk Basbach, Journal of Crystal Growth, v.233 (2001) 837-845, reporting that the addition of high viscosity grades of cellulose ethers, for example methyl cellulose, adversely affects the development of gypsum crystals and strength. Thus, larger additions of cellulose ethers are usually avoided in commercial plasters.
Wallboard, which is used herein to also designate such products as sheathing board, moisture resistant board, type-X board, insulation board, shaft liner, soffit board, backing board, core board, ceiling board, gypsum glass mat board, and paperless wallboard, is typically prepared by mixing dehydrated inorganic materials such as calcined gypsum or stucco with water and pouring the resulting slurry into molds, forms or sheets where it hydrates, hardens and dries. Calcined gypsum powder (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) is usually mixed with water and less than 1 wt % of a variety of additives, for example accelerants. Dissolution of the calcined gypsum powder in the water and a resulting hydration reaction causes crystallization of gypsum crystals (calcium sulfate dihydrate) forming the wallboard core. Application of multi-ply face sheets is usually integrated with the formation of the wallboard core. This is often followed by mild heating to drive off the remaining free (unreacted) water to yield a dry product, having face sheets adhered to gypsum core.
Lukevich et al. (WO 99/54265) discloses a method to produce formed gypsum products by extrusion of an α-gypsum paste (using α-calcium sulfate hemihydrate). It is known that α-gypsum is slow setting and drying; therefore, Lukevich et al. prepares an extrudable paste using a nearly stoichiometric combination of α-gypsum plaster and water; however, addition of a clay rheology modifier and a methyl cellulose binder are added to reduce friability (page 1, paragraph 4). The resulting composition is an extrudable paste having near stoichiometric composition of water and α-gypsum and not a slurry.
Thus, Lukevich et al. teaches away from using β-calcium sulfate hemihydrate, which requires an excess of water over and above the stoichiometric limits taught by Lukevich, to form a slurry that can be extruded and requiring a step of drying (page 1, paragraph 3). Additionally, the extrusion of the nearly dry, non-fluid paste, containing clay as a rheology modifier and a cellulose ether as a binder, results in a plaster product with a much greater density and vastly different microstructure than a wallboard core prepared using a slurry of β-calcium sulfate hemihydrate and water.
Morris et al., U.S. Pat. No. 5,482,551, which issued Jan. 9, 1996, disclose a gypsum-based, extruded construction material with a high modulus of rupture and a method of extrusion processing of the construction material. Morris et al. teach a formulation having a low fraction of water to dry ingredients, including gypsum, clay, perlite, a powdered ethyl cellulose binder/rheology aid and fiberized cellulose paper, such that the mixture is a crumbly, semi-dry extrudable composition that maximizes the wet modulus of rupture. Morris et al. teach that an extruded wall panel must have a high enough wet strength to be self supporting.
However, the extruded construction material of Morris et al., like that of Lukevich, is too dense to be used commercially as wallboard. Even with substantial inclusions of lightweight perlite (16% by weight of the dry ingredients) and near the maximum ratio of water:gypsum allowed by Morris et al. (0.8), the density of the product was still 54.8 pcf (0.88 g/cc). Typical densities were about 69 pcf (1.1 g/cc). These densities are unacceptable for production of commercial wallboard, because the added weight of the wallboard adds significantly to higher transportation, handling, and installation costs compared to conventional wallboard.
Gypsum-based wallboard is used primarily as inexpensive and easily formable coverings with adequate compressive strength, nail pull resistance, flexural strength and good fire resistance. However, even conventional gypsum-based wallboard products are heavy compared to other modem building materials, and this extra weight adds to the cost of production, delivery, installation and disposal of gypsum-based construction materials compared to competing products. Thus, it is desirable to retain the beneficial qualities of gypsum-based wallboard while reducing the overall cost of installed wallboard sheets by reducing the weight of gypsum-based wallboard.
Also, strength of conventional wallboard is related primarily to the strength of the facing paper, typically an oriented fiber, multi-ply facing paper that is applied to the gypsum-based slurry, which forms the core of the wallboard, during a continuous forming process. For a ½ inch wallboard with a density of about 0.6 g/cc, approximately one-half of the nail pull resistance and two-thirds of the flexural strength are supplied by the paper face sheets, which also account for 40% of the manufacturing costs. The core is usually exceptionally poor at handling tensile loads of any kind.
Others have reduced the weight of the core further by adding porosity and/or a low-density, expanded filler (e.g. perlite) into the conventional material. Adding such porosity or filler decreases the density of the core, but also reduces the strength of the wallboard. The strength of gypsum sheets decreases dramatically with density. For example, a dramatic decrease of the nail-pull resistance with density of ½-inch gypsum wallboard, both papered and non-papered, can be seen in FIG. 3.
Typically, the rate of loss in strength is not merely proportional with the reduction in density, but instead the strength-to-weight ratio of the wallboard core decreases with the addition of porosity and/or low-density filler, such as perlite, compared to that of a fully dense gypsum wallboard core. The resulting flexural strength of the wallboard may be acceptable, so long as the strength of the multiply facing is sufficient to offset any weakening of the core, and the reduced core density does not cause the failure mode to change from tensile failure of the facing to crushing of the core. However, nail pull resistance of the wallboard is reduced by addition of such porosity, because increasing porosity rapidly reduces the resistance of the core to crushing and densification. Therefore, the nail pull resistance of the wallboard, which depends greatly on the nail pull resistance of the core, becomes the limiting criterion for wallboard with low-density cores covered by face sheets. For paperless wallboard core, the flexural strength may be the limiting failure criterion, because unreinforced gypsum wallboard cores have little, if any, resistance to the tensile load components in the flexural strength test.
Another way of compensating for the introduction of lower density substitutes (e.g., expanded perlite or air voids) for part of the set gypsum matrix is to increase the strength of the set gypsum above normal levels in order to maintain overall core strength. A number of additives, such as cellulosic particles and fibers, have been included to further improve the mechanical properties of cementitious products. More expensive glass fibers are used in place of wood in applications where high fire resistance is required, such as the shaft liner for elevators. However, conventional fibers, particularly glass, do not adhere well to the gypsum matrix and decrease the workability of the gypsum slurry, thus limiting possible improvements to the core strength. Glass fibers are also brittle and can be easily dislodged during board handling, installation, or demolition to cause irritation of the skin or respiratory tract.
More recently, there has been increasing interest in improving the strength and wear resistance of construction materials by incorporating polymers and/or starches into the core material, although starches are not generally considered strength enhancers. Cementitious composites containing water-dispersible polymers having modest improvement in strength-to-weight have been found by adding latex or other strengthening polymers to the cementitious materials.
However, several unique challenges have thus far restricted the commercialization of polymer reinforced cementitious products to relatively expensive niche products. For example, the nail pull resistance may decrease with the addition of some organic additives or an increase in nail pull resistance may require concentrations of polymers greater than 5 wt %, which can lead to problems such as inflammability, reduced extinguishability, commercially unacceptable cost of the wallboard, and mold susceptibility. Therefore, there is a longstanding and unresolved need for an additive that can increase both the nail pull resistance and the flexural strength of wallboard core, allowing the core density to be reduced.
Cellulose is a polysaccharide composed of individual anhydroglucose units which are linked through a glycosidic bond (FIG. 16). The number ‘n’ of anhydroglucose units in the polymer chain is defined as the degree of polymerisation. Typically, production of cellulose ethers (CE's) involves replacing some of the hydroxyl hydrogen groups of cellulose with a substituent group, for example a methyl group, an ethyl group, a carboxymethyl group, a hydroxyehthyl group, a hydroxypropyl group, or some combination thereof. For example, a hydroxyethyl methyl cellulose (HEMC) may be produced by replacing some of the groups of cellulose with hydroxyethyl groups and methyl groups. Likewise, a hydroxypropyl methyl cellulose (HPMC) may be produced with hydroxypropyl and methyl groups replacing some of the hydroxyl groups of the cellulose.
The number of substituted hydroxyl groups per anhydroglucose unit is expressed as the degree of substitution (DS). The DS can vary between 0 and 3. As with all polymer reactions, this reaction does not occur uniformly along the polymer chain. The reported degree of substitution is therefore a mean degree of substitution over the whole polymer chain. Alternatively, molar substitution (MS) may be used to report the number of moles of substituent groups, such as a hydroxypropyl group, per mole of anhydroglucose. Often, manufacturers follow a convention whereby one of the substituents is reported by DS and the other by MS, where the substituent reported by MS may replace a hydroxyl group or may attach to another substituent in a chain. The DS is not always reported, and we have found that the value reported is often inaccurate or given as a broad range, as shown in Table I.
In another alternative, the weight percent of substituents is reported. Weight percent of substituents may be directly related to DS and MS. For example, the following equations show the conversion for HPMC:                                           DS            ⁡                          (                              OCH                3                            )                                =                                                    wt                ⁢                                                                   ⁢                %                ⁢                                                                   ⁢                                  OCH                  3                                            31                        *                          162                              100                -                                  (                                                            wt                      ⁢                                                                                           ⁢                      %                      ⁢                                                                                           ⁢                                              OC                        3                                            ⁢                                              H                        6                                            ⁢                                              OH                        /                        1.29                                                              +                                          wt                      ⁢                                                                                           ⁢                      %                      ⁢                                                                                           ⁢                                              OCH                        3                                            *                      0.45                                                        )                                                                    ⁢                                  ⁢        and                            EQ        .                                   ⁢        1                                          MS          ⁡                      (                                          OC                3                            ⁢                              H                6                            ⁢              OH                        )                          =                                            wt              ⁢                                                           ⁢              %              ⁢                                                           ⁢                              OC                3                            ⁢                              H                6                            ⁢              OH                        75                    *                      162                          100              -                              (                                                      wt                    ⁢                                                                                   ⁢                    %                    ⁢                                                                                   ⁢                                          OC                      3                                        ⁢                                          H                      6                                        ⁢                                          OH                      /                      1.29                                                        +                                      w                    ⁢                                                                                   ⁢                    t                    ⁢                                                                                   ⁢                    %                    ⁢                                                                                   ⁢                                          OCH                      3                                        *                    0.45                                                  )                                                                        EQ        .                                   ⁢        2            
Cellulose ethers are conventionally differentiated by type of substituent and the viscosity of an aqueous solution of the cellulose ether. For example methyl cellulose (MC), ethyl cellulose (EC), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), ethyl hydroxyethyl cellulose (EHEC), ethyl hydroxypropyl cellulose (EHPC) and hydroxypropyl cellulose (HPC) are named for the type of substituent group used to replace the hydroxyl group in cellulose. The viscosity of an aqueous solution including a cellulose ether is an important characteristic for its typical use as a thickener; therefore, cellulose ethers are also differentiated by viscosity, which depends on the degree of polymerization (directly related to the measured molecular weight), and the type and degree of substitution of substituent groups. As the molecular weight increases, the viscosity of an aqueous solution of the cellulose ether increases also. However, the effect of the degree of substitution depends on the particular type of substituent group, which may also effect the solubility of the cellulose ether.
Manufacturers characterize the effect of a particular cellulose ether on the viscosity by reporting the measured viscosity of a 2 wt % aqueous solution of the cellulose ether. Herein, we refer to this 2 wt % viscosity as the viscosity grade of the particular cellulose ether. Typically, the viscosity grade is measured by one of two techniques: Brookfield and Ubbelohde. Often, the measured viscosity grade differs between the two techniques. For example, results using both techniques are shown in Table I for some cellulose ethers.
Cellulose ethers are not typically used in wallboard products, but may be used at low molecular weights (low viscosity) and low concentrations to provide proper water retention, pumpability and/or increase mixing blade life.